I quit WoW a year ago. I would come back for this (for a weekend.. not perm). Bravo Blizz. As much as I hate you, this is a really awesome event!
Big question is.. "max-level players". If the expansion drops before this, there is no way in hell I'm going to buy it and level to 100 just to get that mount.
So.. Blizz can be greedy and ask for $65 - and get none. This would probably piss me off enough to never consider coming back ever again.
Or they can be nice and I'll gladly hand them $15.